The role of sympathomimetic amines in the dyspnoea of thyrotoxicosis.

Nine untreated thyrotoxic patients (5 males and 4 females) aged 23 to 56 years who complained of dyspnoea on moderate exertion were studied. Measurements of forced expiratory volume (FEV1), vital capacity (VC) and total lung capacity (TLC), oxygen consumption, (VO2), carbon dioxide output (VCO2) and the minute ventilation (VE) were determined on three occasions: before treatment; after treatment with propranolol and after carbimazole. Dyspnoea disappeared in 4 patients out of 9 after treatment with propranolol, while none were dyspnoeic when they became euthyroid. There was no significant difference in FEV1, VC, TLC and FRC on the three occasions. The differences between the VO2, VCO2 and VE before treatment and these values after treatment with carbimazole were significant. We conclude that the dyspnoea of thyrotoxicosis is due to both the effect of catecholamines and the metabolic effect of thyroxine.